Semiconductor nanoparticles with a particle size of less than 10 nm lie in a transition region between bulk semiconductor crystals and molecules. They exhibit physicochemical properties that are different from those of both bulk semiconductor crystals and molecules. In this region, the energy gap of the semiconductor nanoparticles increases with decreasing particle size, due to the emergence of the quantum size effect. This is further accompanied by the removal of the energy band degeneracy that is observed in bulk semiconductors, so that the orbits become discrete, with the lower end of the conduction band shifted to the negative side and the upper end of the valence band shifted to the positive side.
CdS semiconductor nanoparticles can be easily prepared by dissolving equimolar amounts of Cd and S precursors. The same is true for the manufacture involving CdSe, ZnS, ZnSe, HgS, HgSe, PbS, or PbSe, for example.
Semiconductor nanoparticles are gaining much attention because they are capable of emitting strong fluorescence with a narrow full width at half maximum, which makes it possible to produce a variety of fluorescent colors, thereby opening up a great variety of future applications.
However, the semiconductor nanoparticles obtained by simply mixing precursors as mentioned above exhibit a wide particle size distribution, and consequently the properties of the semiconductor nanoparticles cannot be fully utilized. Attempts have been made to accurately conduct particle size separation on semiconductor nanoparticles immediately after preparation that have a wide particle size distribution, using a chemical technique, and then separate and extract only those semiconductor nanoparticles with a specific particle size, with a view to achieving monodispersion. Such attempts include an electrophoretic separation process utilizing the fact that the surface charge possessed by nanoparticles varies depending on particle size, an exclusion chromatography process that takes advantage of the difference in retention time depending on particle size, and a size-selective precipitation process that utilizes the difference in dispersibility into an organic solvent depending on particle size.
The foregoing are examples of techniques for classifying the nanoparticles prepared by mixing precursors according to particle size. Another technique for achieving monodispersion of particle size has also been reported; namely, a size-selective photoetching process that utilizes the oxidizing dissolution of a metal chalcogenide semiconductor upon light irradiation in the presence of dissolved oxygen.
There is yet another method in which monodispersion of particle size is controlled at the stage of mixture of the precursors. One typical example is a reversed micelle method. In this method, amphipathic molecules, such as diisooctyl sodium sulfosuccinate, and water are mixed in an organic solvent, such as heptane, thereby forming a reverse micelle in the organic solvent, such that precursors are reacted with each other using only the aqueous phase in the reverse micelle. The size of the reverse micelle is determined by the quantitative ratio of the amphipathic molecules to the water, so that the size can be relatively uniformly controlled. The size of the thus prepared semiconductor nanoparticle is dependent on the size of the reverse micelle, so that it is possible to prepare semiconductor nanoparticles with relatively uniform particle sizes.
While the semiconductor nanoparticles obtained by the aforementioned processes exhibit a relatively narrow particle size distribution, the fluorescent properties of the thus prepared semiconductor nanoparticles exhibit a gradual fluorescent spectrum without any significant peaks. Moreover, the fluorescent spectrum exhibits a peak at a wavelength that is different from the theoretical value of the fluorescence that is supposed to be emitted by the semiconductor nanoparticles. Specifically, in addition to the bandgap fluorescence exhibited from the inside of the semiconductor nanoparticles, the aforementioned semiconductor nanoparticles emit totally separate fluorescence that is believed to be emitted by energy levels existing in the forbidden band of energy levels of the semiconductor nanoparticles.
The energy levels that emit this fluorescence are believed to exist mainly at a surface site of the semiconductor nanoparticles. This is a phenomenon that adversely affects the properties of the semiconductor nanoparticles with a narrow particle size distribution and has remained a problem to be solved, as the changes in fluorescent properties brought about by controlling the particle size of semiconductor nanoparticles originally appear in bandgap fluorescence.
As a typical solution to the above problem, a method has been attempted that would coat a semiconductor material as a core with a semiconductor material that has a larger bandgap than that of the core's semiconductor material, an inorganic material, and an organic material, thereby forming a multilayer structure in order to suppress the aforementioned fluorescence.
Typical examples of coating with an inorganic material include a coating of a CdSe nanoparticle with CdS (J. Phys. Chem. 100: 8927 (1996)), a coating of a CdS nanoparticle with ZnS (J. Phys. Chem. 92: 6320 (1988)), and a coating of a CdSe nanoparticle with ZnS (J. Am. Chem. Soc. 112: 1327 (1990)). With regard to the coating of a CdSe nanoparticle with ZnS, the Ostwald ripening phenomenon has been successfully utilized to obtain semiconductor nanoparticles with a sufficient fluorescent property by conducting the coating in a coordination solvent (J. Phys. Chem. B. 101: 9463 (1997)).
In the aforementioned multilayered semiconductor nanoparticle, the particle is coated with a material having a larger bandgap than that of the semiconductor nanoparticle and having no bandgap in the forbidden band thereof. This is in order to suppress the defective sites on the surface of the semiconductor nanoparticle so that the inherent fluorescent property of the semiconductor nanoparticle can be obtained.
In a method of surface processing in an aqueous solution, an improvement has been reported in the fluorescent property of the semiconductor nanoparticle in an alkali aqueous solution (J. Am. Chem. Soc. 109: 5655 (1987)). Although various experiments and reports have been made based on this report, none have successfully shed light on the mechanism (J. Phys. Chem. 100: 13226 (1996); J. Am. Chem. Soc. 122: 12142 (2000), for example). Moreover, none of the semiconductor nanoparticles in the alkali solution have sufficient reproducibility, and the reproduction conditions have not been identified. Furthermore, none of the experiments or reports has succeeded in isolating the final product.
As an example of the method for coating with an organic material, a synthesization process can be cited that utilizes the Ostwald ripening phenomena in a coordination solvent. It employs a coating material such as TOPO (trioctylphosphine) or hexadecylamine (HDA) as the coating material, for example, to obtain semiconductor nanoparticles with high light-emission properties (J. Am. Chem. Soc. 122: 12142 (2000), J. Lumin. 98, 49 (2002), for example). It should be noted, however, that the finally obtained semiconductor nanoparticle is not water-soluble.
The semiconductor nanoparticle obtained by the above-described methods is capable of suppressing a defect site to some extent and has the inherent properties of a semiconductor nanoparticle to some extent. However, in order to prepare such a semiconductor nanoparticle, a highly sophisticated technique is required, and in order to achieve high quality, a variety of equipment is required. Further, they are seriously deficient for the purpose of industrial production from the viewpoint of the cost of reagents and safety during high temperature reaction.